1. Field of the Invention
The present invention generally relates to an optical disk used substrate and an optical disk as a large capacity recording medium, and more particularly, to an optical disk substrate and an optical disk capable of being recorded at high density, and accessed from opposite sides or both faces.
2. Description of the Prior Art
Conventionally, the construction of an optical disk capable of being accessed from opposite sides or faces has been disclosed, for example, in Japanese Laid-Open Patent Publication Tokkaisho No. 60-57553, and cross sections thereof are shown in FIGS. 10(a), 10(b) and 10(c).
In FIGS. 10(a) and 10(b), the known optical disk includes substrates 101 each made of a resin which transmits an irradiation beam, an optical subbing layer 102 provided on said substrate 101 for imparting a surface smoothness or forming tracks, etc. thereon, and an active layer 103, a spacer layer 104 and a reflective layer 105 further laminated sequentially on said optical subbing layer 102 as shown.
In FIG. 10(a), the substrate 101 after the lamination as described above are bonded to each other through adhesive resin layers 107, with a mechanical lining 106 of an aluminum plate or the like being held therebetween. Meanwhile, in the arrangement of FIG. 10(b), the substrates 101 after the lamination as stated above are directly bonded to each other through the adhesive resin layer 107. In the construction of FIG. 10(c), after laminating the optical subbing layer 102, active layer 103, spacer layer 104, and reflective layer 105 sequentially on the substrate 101, another spacer layer 108 and an active layer 109 are further laminated to achieve an optically mirror symmetrical relation at the reflective layer 105, with an overcoat layer 110 being further provided as illustrated.
FIG. 11 is a schematic side elevational diagram showing an optical disk device of a dual sided access type, in which a double-sided disk 111 has a recording capacity two times that of a single-sided disk, and is capable of being simultaneously accessed on front and reverse faces by the optical disk device. More specifically, the double-sided disk 111 is mounted on a rotary shaft 113 of a motor 112 for rotation in a predetermined direction, and by read/write heads 114 and 115 provided at the front and reverse face sides of the double-sided disk 111, reading and writing of information are effected with respect to each face of the disk 111.
Moreover, as one of the means for improving a recording density of an optical disk, there has conventionally PG,4 been proposed a method for forming neighboring tracks alternately into lands and grooves as shown in FIG. 12, in which a single-sided disk 116 has its tracks formed by the lands and grooves. For reading and writing information, as observed from an incident side of light, the read/write head 114 accesses the track for the groove, while the read/write head 115 accesses the track for the land, by which arrangement, the recording capacity may be doubled as compared with the case where only the lands or grooves are utilized for the track.
In Japanese Laid-Open Patent Publication Tokkaihei No. 2-189743, there is disclosed a system in which, in a dual sided access type optical disk, with respect to the neighboring tracks formed with the lands and grooves as in the single-sided disk 116 in FIG. 12, the track for the land is accessed from the front face, while the track for the groove is accessed from the reverse face.
As shown in FIGS. 10(a) and 10(b), in the conventional double-sided disk, since the information of the track and address is provided in the substrate 101 or the optical subbing layer 102, such information of the track and address is completely independent on the front and reverse faces. However, due to the manufacturing process to bond two single-sided disks to each other after preparation thereof, many man-hours are required for the manufacture, thus resulting in a cost increase. Meanwhile, when the optical disk as shown in FIG. 10(c) is adopted, there is an advantage in that the labor time is reduced for cost reduction. On the other hand, in the type where both faces of the optical disk are simultaneously accessed, since rotating directions of the optical disk are opposite in the front and reverse faces, if the optical disk has the constructions as explained with reference to FIGS. 10(a) and 10(b), it becomes necessary to provide the substrate or subbing layers of different formats. In order to solve such a problem, there has been proposed, for example, in Japanese Laid-Open Patent Publication Tokkaihei No. 4-64933, an arrangement in which two address rows are provided in pairs so that one address may be read in a direction opposite in the scanning direction, but in this case, the length of the address becomes undesirably longer.
In the case where the substrate for the conventional single-sided disk is to be accessed from both sides, heat interference occurs between the tracks on the front and reverse faces, if the same tracks are used for the front and reverse faces. For example, in the optical disk of a rewritable type, there is a problem that a recording mark disappears.
Such heat interference will not take place when different tracks are employed on the front and reverse faces, but conventionally, since pits for the address information are located at approximately the same position, there has also been such a problem in that the information of the address is not easily read or tracking becomes unstable due to interference with pits on the neighboring tracks.
Moreover, in the conventional practice, one track is divided into a plurality of sectors for allocation of the addresses, and in this case, since the neighboring tracks are located at approximately the same position, a waiting time for at least one sector is required for the change-over of data on the front and reverse faces.
Even in the case of the one side access, there has also been an inconvenience in the conventional arrangement in that, in the optical disk utilizing both of lands and grooves for the tracks, since the pits for the address information are located at approximately the same position, interference occurs with respect to the pits of the neighboring tracks, thus making it difficult to read the information of the address or making the tracking unstable. Similarly, the problem that the waiting time at least for one sector is required for the change-over of the data between the lands and grooves, is also involved in this case.